Woodpulp is manufactured in the pulp and paper industry by various methods. Pulp can be manufactured chemically by cooking or mechanically by grinding and refining. It is also manufactured from waste paper by defibering the waste paper in a pulper. It is common to all pulp manufacturing methods that the pulp contains more or less impurities which must be removed therefrom. In a general sense, pulp is "cleaned" in a screening plant by means of screens and cleaners. A screen is an apparatus in which pulp in the consistency range of about 1 to about 5% is cleaned by either a slotted screen or a perforated screen. The cleaner, normally a so-called centricleaner, is an apparatus in which pulp is cleaned by centrifugal force in a low, usually less than 1%, consistency. However, cleaning involves two major problems. First, it is usually desirable after cleaning to increase the pulp consistency to a range of about 10 to about 15% for storing or after-treatment. Secondly, handling of the reject is also desirable by either refining or some other method, but usually at a higher consistency than that present during cleaning. In other words, the pulp flows must always be thickened after cleaning. There have been attempts to resolve this problem such, for example, by the Swedish company Kamyr AB. Their solutions aim at raising the consistency to 8-15% in the cleaning equipment. Efforts have been made in developing both screens and cleaners which will operate at a consistency of about 10%. However, this has been only partially successful. Screening and centrifugal cleaning as such can be performed rather successfully at a high consistency, but the separation efficiency of the screens and cleaners is substantially decreased as the consistency increases. It can thus be said that Kamyr has replaced one problem with another, i.e. they have eliminated the need for thickening at the cost of cleaning efficiency.